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Locks a variable or a subroutine. A lock on a variable is maintained until the lock goes out of scope. If the variable is already locked by another thread, the lock call blocks until the variable is available. You can recursively lock the variable, which stays locked until the outermost lock goes out of scope.

Note that locks on variables only affect other lock calls; they don't prevent normal access to a variable. Also, locking a container object (e.g., an array) doesn't lock each element of the container.

Locking a subroutine blocks any calls to the subroutine until the lock goes out of scope; no other thread can access the subroutine while the lock is in effect.